Measuring transmission delay



H. NYQUIST MEASURING TRANSMISSION DELAY Filed Sept. 1'. 1925 INVENTOR' Eli gnaw 7 BY f v I L ATTORNEY Patented Aug. 24, 1926.

UNITEDSTATES PATENT. OFFICE.

HARRY NYQUIST, OF MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE- AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

MEASURING TRANSMISSION DELAY.

Application filed September 1, 1925. Serial No.53,942.

, It is an objectof my invention to provide a method and appropriate apparatus for measuring the delay in transmission of an alternating current through a given line or one comprising a transducer whose delay is to be measured; by reversing the connections in one path and varying the frequency,

of the oscillator, two consecutive adjustments can be found giving a null effect in the receiver and the difference of delay between these two adjustments will be 180. From this datum and from the known change in frequency of the oscillator the delay can readily be computed; These objects and other objects of my invention will become apparent on consideration of an eX- ample of practice according to the invention which I have chosen for disclosure in the following specification, takenwith the accomanying drawing. It will be understood that the following description relates to this example of the invention and that the invention will-be defined in the appended claims,

\ and transducer Q, t0'.th8 receiver R0.

Referring to the drawing, this is a diagrammatic representation of sociated apparatus that may be employed in the practice of my invention.

By closing the switch S, the oscillator 0 will be operatively connected through the transformer T artificial line AL switch S Also the oscillator 0 can be connected inthe same way through-the switches S and S and each oscillator 0 or 0 can be connected through the transformer T more directly with the receiver R0. By means of the switch S the artificial line AL can be included-in either of these two paths from the oscillators to the receiver R0.

The currents from the oscillators O and or either of them may also be taken through, the potentiometer P and switch S, to a detector D. Further details of structure will be pointed out in connection with the following description of the use of the ap aratus shown in the drawing.

& is a transmission line or any transcircuits and asducer for which it is desired to determine the delay of transmission at a particular frequency or at various frequencies. It is connected in the system as shown and the oscillator 0 is adjusted to give a current of a suitable frequency, say about 1,000 cycles per second, if the investigation has reference to the voice frequency range. Switches S and S are closed and S and S are left open. Switch S is thrown to the right or .the left according as the transducer Q gives a net loss or a net gain.

The frequency of the oscillator 0 is then carefully adjusted and the artificial line AL is also adjusted soas to get a null effect in the receiver Be. At this adjustment, it

means that currents arriving at the receiver from the oscillator 0 over the two paths are of equal intensity and'180 (or an odd multiple thereof) out of phase. The two paths mentioned are one of-them through the transformer T artificial line AL and transducer Q, and the other path is through the transformer T Next, close the switch S and reverse the switch S thus the receiver Re Will get. sound from both oscillators ,O, and 0,.

Adjust the oscillator 0 until its frequency comes somewhere near that of the oscillator 0 and beats will be noticed in the receiver R0. Further adjust the oscillator 0 to the no-beat adjustment; this means that the two oscillators O and 0 now have the same frequency. Call this frequency f,; its numerical value is not as yet known.

Now restore the switclrS to its former position and open the switch S It is apparent that there should be no sound in the receiver R0, and this will be the case if the transformers have been properly poled; however, if this is not the case, and if aloud sound is heard, it may be overcome by throwin the. reversing switch S hlow throw the reversing switch S whereas the currents to the receiver R0 over the two paths were formerly in phase opposition in their effect, they Wlll now be in phase reinforcement and a loud tone will be heard in the receiver R0. Slowly adjust the oscillator 0 in frequency until this tone, disappears in the receiver R0. This will mean that with the phase angle of the apparatus Q a function of frequency, the frequency has been changed enough to shift the phase angle 180'. Call, this second frequency f,. In

order to get the zero effect at this frequency it may be necessary slightly to readjust the artificial line AL \Ve now have the two oscillators and 0 adjusted at respective frequencies f and f, such that the transmission delay through Q differs by 180 for these two cases. We next are to determine the magnitude of this frequency difference.

With the switches S S S and S closed, currents of these two frequencies Will be superposed in the input of the detector D. The polarized relay PR in the output circuit of this detector D will operate at each beat between the two frequencies f, and f causing the operation of the sounder N. This sounder will give a rough idea of the number of beats per second and will thereby indicate whether or not the milliammeter M is liable-to injury when it is thrown into the circuit. Having determined this, open switch S Thereupon the operation of the polar relay PR will cause a deflection of the -milliammeterM This deflection, together with acalibration curve, may be used to give the count of the beats per second. The number of beats per second will be equal to e formula TZdfi/dw has been found to hold with suflicient accuracy. In this formula where T is the delay in transmission, 6 is the phase displacement in radian measure and (0221: times the frequency. In.

other words, the delay equals the increment of phase angle divided by' the increment of periodicity. In the case under consideration the increment of phase angle is 11', and the increment of periodicity is 21r(f f and their -quotient is =1/[2(f --f The foregoing ormula gives the delay at a frequency which can be taken as the,

mean of f and f The delay at other values of frequency can be obtained in a similar manner and thus a plot can be made of the delay as a function of frequency.

Instead of measurin the frequency difference f,-;", by the eat method as here described, the measurement may be made by means of abridge B. In this case the switch S is shifted from the position shown in the drawing and the beat currents from the detector D go through the filter f to the bridge B, three arms of which are resistances, but the fourth arm has an inductance and; capacity in addition to a resistance. For. any beat frequency this inductance and capacity may be adjusted to resonance, thus giving a null effect in the meter M The inductance and capacity are calibrated so that from their adjustment the beat frequency can readily be determined.

i I claim:

quency f at which the difference between said currents is Increased or decreased by 7rand getting the delay by the formula T: /l; (f27f1)l' The method of measuring the delay of transmission through a given transducer at a certain frequency, which consists in receiving the input and output currents ina common receiver and adjusting the freuency to zero tone in the receiver whereby t 1e phase difference is known to be an odd multiple of 71', then reversing one current to the receiver and changing the frequency to the next value giving zero tone, then getting the diiference between these two frequencies and gettin the delaytby, the formula T:1/[2(f 4. Means to measure the delay through a.

the frequency of a source and determining the change of frequency that gives a half,

period change of phase shift through the transducer.

6. The method of measuring the delay of transmission through a 'ven transdueer at a certain frequency whic consists in changing the frequency over a narrow range and noting the corresponding change of phase shift through the transducer and deducing the delay therefrom.

7. The method of measuring the delay of transmission through a given transducer at a certain frequency which consists in chang ing the frequency enough to change the phase shift by a definite easily ascertained angle, and from these data deducing the delay.

In testimony whereof, I have signed my name to this specification this 26th day of August, 1925.

HARRY NYQUIST. 

